The invention may be carried out by an independently operated machine i.e. which carries out bale wrapping only, or by an integrated combined baler and wrapper apparatus.
It is well known to apply stretch film wrapping to enclose agricultural bales tightly, and which may be composed of e.g. straw, hay or partly dried grass (to ferment when wrapped to form silage). Bales are usually of cylindrical shape, so-called big bales or round bales, or are of rectangular cross section (which includes square cross section). The latter are more difficult to wrap satisfactorily, by reason of the eight corners of the bale which create problems in application of stretch film wrapping when a film strip passes from planar coverage of one flat surface of the bale to an adjoining flat surface via one of the corners.
This is usually not a problem in passing from an end face to a cylindrical outer surface of a round bale, or vice versa, in that the circular edge of the round bale provides an easier transition for the film strip from one flat surface to another.
By contrast, a corner of a rectangular bale provides a smaller area only over which a small portion only of the film strip engages at high pressure, as the film strip bends through 90 degrees approximately in moving from one flat surface to another. Stretch film used to wrap bales undergoes a pre-stretching action, prior to application to the bale, and the elastic energy thereby stored in the film strip enables the stretched strip to tighten itself onto the outer surface of the bale, after application to the bale. However, while this provides satisfactory wrapping of the major part of the flat surface of the bale, there is usually a problem in providing satisfactory coverage at each corner region of a rectangular bale using conventional wrapping.
The problem which arises, using conventional wrapping techniques, when a winding which is being wrapped around a bale of rectangular cross section, and moves from one planar face of the bale to another via one of the corner regions of the bale. The winding is of substantial width, and (as will be described in more detail below with reference to FIG. 1), one of the longitudinal edges of the winding has a longer path of travel than the opposite edge portion, and therefore the edge portion which has the greater distance to travel undergoes greater elongation during the wrapping process, whereas the opposite edge portion which has the shorter distance to travel is under less tensile stress, or even zero tensile stress, and therefore tends to form wrinkles or creases in the winding which extend laterally inwardly from the edge which is required to travel the shorter distance.
Clearly, this is undesirable, in that one edge portion of the winding is under high tensile stress, whereas the opposed edge portion is under less or zero tensile stress, and there is possible risk of rupture or tearing of the edge portion under high tensile stress and/or undesirable generation of creases or wrinkles in the opposed edge portion, which has an adverse effect on the air and liquid tightness of the wrapped bale, after the wrapping process has been completed.